1. Field of the Invention
Embodiments of the invention generally relate to a method and apparatus for effectively separating hydrocarbons from an acid gas stream by treating the gas in at least one distillation zone containing a dividing wall and a controlled freezing zone.
2. Description of the Related Art
Sour natural gas is treated to remove carbon dioxide (CO2), hydrogen sulfide (H2S), and other contaminants before it can be further processed to recover natural gas liquids, used as an environmentally-acceptable fuel, or used as feedstock for a chemical or gas to liquids facility. For “highly sour” feeds (e.g., >20% CO2+H2S), it can be particularly challenging to design, construct, and operate an economically viable process for separating these contaminants from the desired hydrocarbons. In many cases, the concentrated acid gas (consisting primarily of H2S and CO2) is sent to a sulfur recovery unit (SRU) to convert the toxic H2S into benign elemental sulfur, while the CO2 is vented.
Sulfur recovery is a relatively expensive and complex process. In some areas (e.g., the Caspian Sea region), additional elemental sulfur production is particularly undesirable because there is no market for it. Consequently, millions of tons of sulfur are stored in large, above-ground blocks in some areas of the world. Venting of CO2 is also coming under greater scrutiny, as some countries have ratified the Kyoto protocol requiring reduction of CO2 emissions. Thus, acid gas injection (AGI) is often a preferred alternative to sulfur recovery.
Cryogenic gas processing (i.e., distillation) avoids the use of solvents, minimizes acid gas removal equipment, and generates a liquefied concentrated acid gas stream at moderate pressure (e.g., 350-500 pounds per square inch gauge (psig)) that is suitable for injection into a subterranean reservoir. Because the liquefied acid gas has a relatively high density, hydrostatic head can be used to great advantage in an injection well. The energy required to pump the liquefied acid gas is much lower than that required to compress low-pressure acid gases to reservoir pressure. Cryogenic processing also requires fewer stages of compressors and pumps.
If CO2 is present at concentrations greater than about 5% in the gas to be processed, the CO2 freezes out as a solid in a standard cryogenic distillation unit. This renders normal cryogenic distillation processes inoperable. The Controlled Freeze Zone (CFZ) process circumvents this problem by deliberately freezing the CO2 out in a specially-designed distillation tower. The CFZ process and a variety of improvements are described in U.S. Pat. Nos. 4,533,372; 4,923,493; 5,062,720, 5,120,338, 5,265,428, and 6,053.007. The CFZ process generates a clean methane stream (along with any nitrogen or helium present in the raw gas) overhead, while a liquid acid gas stream at 30-40° F. is generated at the bottom of the tower. The CFZ process is thus synergetic with AGI.
However, any hydrocarbons heavier than methane are lost with the acid gas. Therefore, there is a need for a method and apparatus for separating and recovering additional hydrocarbons from a liquid acid gas stream.